1. Technical Field
The present invention concerns piston pin bushings.
2. Related Art
The present invention concerns a piston pin bushing in accordance with the introductory portion of patent claim 1.
Piston pin bushings are known for example from U.S. Pat. No. 4,909,638 and DE-OS 28 18 378, both of which deal with the manufacture of bushings, in particular with notching. Such bushings are used for housing the connecting rod eye in combustion engines with pistons having an up and down motion. Both parallel bushings and trapezoid bushings are described that can feature oil ducts, as well as oil grooves appearing on the inner surface. These oil grooves, moreover, represent a material weakness; but they are simply inserted in the sleeve bearing material attached on the steel backing with the result that the material weakness of the entire bushing is so small that it has no impact on the radial pressure. The same applies to the oil ducts, whose diameter is so small that the elastic and plastic qualities of the bushing are not affected. Furthermore, the oil grooves extend over the entire interior circumference.
In FIG. 1A the installation position for a parallel bushing 20, 20a is represented schematically which also applies to the bushing yet to be explained according to the present invention. Piston 2 executes an up and down motion in cylinder 1 that is transferred to connecting rod 5 and lifting pin 10 mounted in large connecting rod eye 7 of the crankshaft. Connecting rod 5 contains a push rod bushing in small connecting rod eye 6, in which bushing the piston pin is housed that extends with both ends in both piston bores 3.
In FIG. 1B piston pin bushings 20b, 20c are designed essentially trapezoidal in the longitudinal view for heavy-duty engines in order to be able to better receive larger forces occurring during the downward motion of the piston, whereby the axially longer section of the bushing lies in the area of maximum load during operation and the axially shorter part of the bushing faces the piston. In using bushings with such beveled ends, a bearing surface can be achieved in this application that is larger in the direction of main load and therefore more effective than those bushings whose ends run perpendicular to the bearing axis of the bushing. With such bushings, both end surfaces, as well as only one end surface, or even only one section of the end surface, can be beveled to the longitudinal axis of the bushing. In FIG. 1B, such half trapezoid piston bushings 20c are used in both piston bores 3, in which the piston pins are no longer directly housed in the piston bores but in both piston pin bushings 20c. 
Piston pin bushing 20, 20b is installed in the small end connecting rod eye 6 by press-seating so that, on the basis of contact, the piston pin bushing exerts a radial pressure on the connecting rod bore which runs a roughly parabolic course in the axial direction, as is shown in FIG. 2 for the realms of both apex lines 23 and 24 by curves I and II.
The contact pressure diminishes at the edge where the piston pin bushing 20 is most loaded during downward motion of piston 2 under consideration of elastic deformation of the piston pin, as is shown for clarification in FIG. 3 in exaggerated representation. Piston pin 4 is thus somewhat deflected by the pressure of the piston so that piston pin bushing 20 is most loaded at the edge, whereby both ends 8 and 9 of the small end connecting rod eye 6 are deflected downwards.
A relative motion between bushing body and connecting rod eye leads, hereby, to friction corrosion. If the load is removed, the bushing springs back to its original position and lifts from the connecting rod body so that in these places oil can penetrate under the bushing body. Both effects result in a diminished lifespan of the piston pin bushing.
Since the maximum cylinder pressure has been increased in automobile manufacture due to direct injection and turbo supercharging in diesel engines in order to reduce fuel consumption or to achieve the prescribed emission values, the load of the piston pin bushing and the connecting rod in the connecting rod eye has increased. The result is that friction corrosion has appeared on the edge of the axially longer section of the piston pin bushing in the rear while crack formations have appeared on the interior, which harm the life-span of the piston pin bushing.
Although it had been attempted in the past to improve conditions by special processing of the bore, precision installation, and optimization of the geometry of the connecting rod eye, problems have not yet been satisfactorily solved.
Proceeding from this knowledge, it is the task of the present invention to create a piston pin bushing that exhibits a sufficient lifespan even at higher piston pressures.
This task will be solved with a piston pin bushing in which the area of material reduction is limited to at least one transition region between the apex lines and that the bushing material is reduced in this transition region at least to the extent that a portion of the piston pin bushing""s radial pressure in the area of the apex lines is shifted in its inserted position from the middle to the area of both bushing ends. that the bushing material is reduced in this transition region at least to the extent that a portion of the piston pin bushing""s radial pressure in the area of the apex lines is shifted in its inserted position from the middle to the area of both bushing ends.
It has been shown that crack formation and friction corrosion are clearly diminished by these methods, which can be traced back to the fact that the contact pressure distribution in the area of the apex lines is changed by the material reduction in a such way that no motion occurs in the longitudinal direction of the bushing on the one hand and, on the other, that the bushing does not prematurely spring back during removal of the load. The bushing body clings to the connecting rod through the increased radial pressure in the end area of the bushing so that relative motions cannot occur.
In order not to impair the surface of the piston pin bushing, the material reduction is to be limited to a transition region between both vertical lines, in which, by reasons of symmetry, the methods in accordance with the present invention are implemented preferably in both possible transition regions between both vertical lines.
Preferably, the transition region on the interior side and/or the bushing exterior side can have at least one pocket. The two dimensional extension and form of the pocket as well as the depth of the pocket or pockets must be selected in such a way that the radial pressure increase according to the present invention occurs in the area of the bushing ends. Conventional pockets are simply put in the slide bearing material. According to the present invention, it is, however, an advantage if these pockets, which are provided on the bushing interior side, extend to the back of the bushings, because in this way the desired material attenuation can be achieved.
In manufacturing the pockets, the bushing material must be detached, by which is understood both the material of the bushing back and the slide bearing material. A pressing procedure for the manufacture of such pockets is not suitable because the material displaced from the pockets leads to a stiffening and thereby to an undesired impact on the elastic and plastic qualities of the bushing material, so that the radial pressure in the area of the bushing ends cannot be increased.
The material diminishment for the purpose of exceeding the flow limit in the inserted position of the bushing, i.e., with conventional press-seating in the connecting rod eye, can proceed until the bushing material is completely detached. The bushing can exhibit not only pockets, but also perforations, whereby pockets and perforations can be combined. These combinations include the possibility that the perforations are provided in the area of the pockets or outside of the pockets.
Concerning the size and the design of such perforations, the same criteria apply as for pockets.
Preferably the transition region extends in the direction of circumference over a 100xc2x0 sector. The transition region is advantageously distant from one of the apex lines between 30xc2x0 and 50xc2x0.
The piston pin bushing can be a parallel bushing or a trapezoid bushing, whereby such bushings under trapezoid bushing are understood as those that simply have a sloped end. Trapezoid bushings are installed in such a way that the first apex line lies in the axially shorter section and the second apex line lies in the axially longer section.
In the trapezoidal bushings the transition region or regions are fitted to the trapezoid form in such a way that the width of the transition region grows in the direction from the first apex line to the second apex line. While the transition region is detached in the parallel bushing at least 12/4 from the edge of the bushing, the dimensions are oriented according to the lengths of the various long apex lines. With a trapezoid bushing, the transition region is thus preferably distant from the edge of the bushing at least 11/4 near the first apex line and at least 12/4 near the second apex line.
This leads preferably to a triangular arrangement or design of pockets and/or perforations, whereby the number and form of pockets and/or perforations can be selected at will. The pockets and/or perforations can lie in the area of the oil grooves or be connected with them so that the pockets and/or perforations can serve as oil reservoirs, which afford an additional positive result.
Embodiments that exemplify the design of the present invention are further explained by the following illustrations.